Readally

"Readally" is a portmanteau combining "read-only" and "read all".

Similar to bindfs, Readally is a FUSE FileSystem that exposes an altered version of a given directory (aka "the original directory").

Specifically, Readally makes it:

• 100% read-only: any attempt to write or change anything is met with errno 30, i.e. EROFS: Read-only file system;
• 100% readable: although each file retains its original owner, group and mode, any file can still be read by any user -- essentially, standard Unix permissions are ignored.

What for?

Unprivileged backup is one possible use case: the process that backs up your data no longer needs to run as root to read the entirety of a given filesystem.

Solutions like bindfs or ID-mapped mounts also allow this but they alter perceived file ownership, which is not always desirable.

Is this not dangerous?

Anything that alters file ownership and/or the behaviour of Unix permissions is dangerous. From this perspective, Readally is as dangerous as bindfs or ID-mapped mounts.

Consequently, these solutions should be used with caution. A typical approach is to protect the mountpoint's parent directory with regular Unix permissions reflecting who is allowed to access the dataset exposed through Readally.

Example:

drwxr-xr-x root   root   /
drwxr-xr-x root   root   mnt
dr-x------ backup root   only_backup_shall_pass
drwx------ root   root   readally_mountpoint
-rw------- root   root   actual_data

Options

one-file-system

Similar to find's -xdev and du's -x, --one-file-system, this option makes Readally ignore any file related to a filesystem other than the one holding the original directory.

Default value: disabled.

banned-types

This option makes Readally ignore a given list of filetypes. Here, filetypes are neither file extensions nor MIME types but rather find-like file types:

Filetypes you likely want to keep: - f: regular files - l: symbolic links

Filetypes you likely want to ignore: - b: block devices - c: character devices - p: named pipes / FIFOs - s: sockets

Alien filetypes: - D: Solaris Doors - P: Solaris event ports - W : whiteouts - ?: unknown

Default value: bcpsDPW? i.e. by default Readally exposes only directories, regular files and symbolic links.

Implementation

• Python with fusepy

How to use it

readally [-o OPTIONS] [--foreground] /original/directory /mount/point

fstab syntax:

/original/directory    /mount/point    fuse.readally    banned-types=DPW?,one-file-system    0 0